One of the major costs in powder metal parts production, as well as other production performed by presses and coining/stamping devices, is that associated with damage to the press and tooling due to overloads. Oftentimes, these overloads are caused by previously formed parts being in the die area when the next part is formed. For example, this can occur if a formed part sticks to the upper punch of the press during ejection and then is brought directly back into the die cavity on the next press stroke.
Further, overloads to the press and tooling may be caused by a blockage of the stream of parts flowing from the press, which forces the last formed part to topple back into the die area. In some ways, this occurrence causes even more catastrophic results since this type of overload places eccentric forces on the tooling and press. Other scenarios which cause overloads include when the feeder mechanism fails and the powder shuttle (or other similar means) remains over the die area and when a part removal system (such as a robot or pick-and-place device) fails in the pick-up location or drops a part after picking it from the cavity.
Most earlier attempts at overload protection by the prior art have dealt with the problem in a gross manner, wherein the press was designed to be protected but not necessarily the tooling associated therewith. Examples of this type of protection include hydraulic capsules in the upper ram, shear plates in the driving mechanism, and hydraulically loaded tie rod nuts. These types of protection systems react directly to an overload on the press, stopping the machine and preventing a second stroke. In most of these cases, however, the tooling is already damaged or destroyed during the measured overload situation before the press could be stopped. In fact, the very nature of a mechanical press, which allows inertial forces to form parts at high speed with minimal horsepower input, makes it likewise impossible to stop the press instantaneously. Photoelectric cells have also been utilized with some success to detect improperly placed parts, but this type of system is relatively difficult to set up and keep clean, somewhat ineffective on very thin parts, and cannot easily be used to detect parts stuck to the upper punch.
More recently, vision systems have been developed for visual inspection applications of this type. The original cameras utilized in such vision systems were of the vidicon type, which are similar to those used in commercial videos. These cameras had a photoconductive surface on the end of the tube which was subject to "burn-in" problems when the camera was used to view a stationary field of vision. Also, the cameras had a very limited life due to shock and vibration problems inherent in applying them to a press. Programming these early systems was also difficult and the associated computer hardware was cumbersome and relatively slow. Moreover, lighting levels were particularly critical, especially when recognition of specific parts in their orientation was concerned.
During the early 1980's solid state cameras were developed using either charge-coupled device (CCD) or charge-injected device (CID) image sensors. These sensors are fabricated directly on silicon chips using integrated circuit technology, and consist of matrix arrays of small, accurately spaced photosensitive elements. When light passing the camera lens strikes such an array, each detector converts the portion of light falling upon it into an analog electrical signal. The entire image is thus broken into an array of individual picture elements known as "pixels." The magnitude of the analog voltage registered for each pixel is directly proportional to the intensity of light in that portion of the image. This voltage represents an "average" of the light intensity variation over the area of the pixel.
In general, solid state cameras offer several important advantages over vidicon cameras when utilized in the press environment. In addition to being smaller in size (recent design advancements include providing remote heads as small as 1 1/4".times.2 1/4", allowing placement of cameras in difficult to access locations), they are much more rugged than the vidicon units. Further, the photosensitive surfaces in solid state cameras do not wear out with use as they do in vidicon cameras, thereby allowing the cost of these units to continue to decline.
Moreover, the vision systems employed today are complex and expensive, which is necessitated by their use in making specific identifications of parts and the like. Since the vision system of the present invention is designed only to recognize if some radical change in the gross or total light level in a scene is observed, it is simpler and less expensive than vision systems of the prior art.